1. Field of the Invention
The present invention relates to a wafer heating apparatus for use in semiconductor producing systems, and methods for producing the same.
2. Related Art Statement
Heretofore, chlorine series gases, fluorine series gases, and similar corrosive gases have been used as deposition, etching, or cleaning gases in semiconductor producing systems which necessitate super clean states. For that purpose, when a conventional heating apparatus having a heat-generating resistive element coated with stainless steel, inconell, or the like, metal is used as a wafer heating apparatus at a state of the wafer contacting with such a corrosive gas, the metal coated on the heat-generating resistive element produces undesirable particles of chloride, oxide or fluoride of diameters of a few .mu.m due to exposure to the gas.
Therefore, a wafer heating apparatus of an indirect heating system has been developed which provides an infrared radiation arranged outside of a container wherein a wafer is exposed to the deposition gas, etc., a window on an outside wall of the container for permeating an infrared radiation emitted from the lamp therethrough into the container, and a heating body made of a highly corrosion resistant material, such as graphite, for irradiating a wafer mounted on the upper surface of the heating body by the infrared radiation to heat the wafer. However, as compared with direct wafer heating apparatuses, this type of wafer heating apparatus has problems in that a large heat loss and a long heating time are required for temperature elevation, permeation of the infrared radiation is gradually prevented by a CVD film deposited on the window which causes the window to heat due to heat absorption thereof, and homogeneity and response of heating become bad due to the separate provision of the heating source lamp and the heating body mounting the wafer.
In order to solve the above problems, we studied arranged on a wafer heating apparatus having a heat-generating resistive element embedded in a dense ceramic disc. As a result, we have found out that this type of wafer heating apparatus is a superior apparatus which eliminates the above problems. However, we have also found out by further studies that there still remains a problem in holding and fixing the semiconductor wafer to the heating apparatus.
Namely, as conventional techniques of fixing a semiconductor wafer, there are known a mechanical fixing system, a vacuum chuck system, and an electrostatic system, which are used, for example, in transportation, light-exposure, film formation, fine machining, rinsing, dicing, etc., of semiconductor wafers. Meanwhile, in heating a semiconductor wafer under a controlled temperature at CVD, sputtering, epitaxial or the like, film preparing, processes, productivity, i.e., yield of production of semiconductors is decreased at the time of producing the semiconductor wafers, if a surface temperature of the semiconductor wafer is not heated homogeneously. In such a case, we have found out that, if a mechanical fixing system is adopted for fixing the semiconductor wafer, the formed film becomes uneven due to contact of a pin or a ring to the surface of the semiconductor wafer. If the semiconductor wafer is arranged at a heating surface of a disc-shaped ceramic heater, the whole surface of the semiconductor wafer is not uniformly urged against the flat surface of the heater disc, so that the semiconductor wafer is deflected and distorted to form a local gap between a portion of the semiconductor wafer and the flat heating surface of the ceramic heater disc. In a medium or a high vacuum of a pressure of not more than 10.sup.-3 Torr, a heat transfer due to gas convection is minor, so that a very large temperature difference is caused between the gap-formed portion of the semiconductor wafer and the portion of the semiconductor wafer contacting the surface of the ceramic heater disc. Namely, behavior of gas molecules at the wafer-mounted surface is in a viscous flow region at a pressure of not less than 1 Torr to allow heat flow (heat transfer) of the gas molecules. Therefore, the wafer temperature is not so considerably decreased as compared with the heater temperature even at the portion having the above gap, and shows a good followability or response to the heater temperature. However, at a medium or a high vacuum range of not more than 1 Torr, the behavior of decreased number of the gas molecules changes to a molecules-flowing region to widely decrease the heat flow due to the heat transfer of the gas molecules, so that the wafer temperature is decreased as compared with the heater temperature to worsen homogeneity and the response property of the heating.
If a vacuum chuck system is adopted for fixing the wafer, the system can not be used for sputtering, CVD or the like processes which are used under a condition of medium or high vacuum.
Though there is an electrostatic chuck using a polyimide film as a dielectric film in the so-called electrostatic chucks, a usable temperature range of conventional electrostatic chucks is around 80.degree.-200.degree. C. at the maximum. Therefore, the electrostatic chuck can not be adopted for fixing the wafer in a sputtering or a CVD process wherein the wafer is heated by a heating element up to a temperature of about 600.degree. C.